Chlorinated elastomers may be cured by numerous means including the use of peroxide/coagent systems, thiadiazole-based systems, or irradiation crosslinking techniques. Peroxide cures are typically preferred for their scorch safety, shelf-life or bin stability, low permanent set, and high temperature performance. However, peroxide cure systems are often unacceptable for use in molded goods because of mold sticking and fouling due to volatiles or in applications that require a low temperature cure due to equipment or processing limitations.
Thiadiazole-based cure systems provide certain advantages such as the ability to cure over a wider range of temperature and pressure conditions than peroxide cures while generating fewer volatile byproducts, having good mold release characteristics, and the ability to use less expensive compounding ingredients such as aromatic oils.
U.S. Pat. No. 4,128,510 discloses that halogen containing polymers can be crosslinked by employing a cure system including 2,5-dimercapto-1,3,4-thiadiazole (or its derivatives) and a basic material such as a metal oxide, metal hydroxide, or metal carboxylate. It further discloses that the addition of an accelerator containing aliphatic or aromatic amines or quaternary nitrogen groups may be desirable; additionally, the condensation product of butyraldehyde and aniline is described as being a particularly useful accelerator.
U.S. Pat. No. 4,288,576 discloses 2,5-dimercapto-1,3,4-thiadiazole (and derivatives thereof) curing systems for saturated halogen containing elastomers, e.g. chlorinated polyethylene, wherein the curing systems specifically include certain vulcanization accelerators. Classes of accelerators include: 1) amines having a boiling point above 110° C. and pK values below about 4.5; 2) salts of amines with pK values below 4.5 and acids having pK values above 2.0; 3) quaternary ammonium hydroxides and their salts with acids having a pK above 2.0; 4) diphenyl- and ditolyl-guanidines; and 5) the condensation product of aniline and mono-aldehydes containing one to seven carbon atoms. The presence of at least an equal amount of an inorganic base is additionally required.
Despite the excellent vulcanizate properties obtainable with the polymercapto/inorganic base/accelerator-based vulcanization systems for chlorinated elastomers, two key problems remain that limit their usefulness. The first problem is the premature vulcanization of the compound during storage (i.e. poor bin stability) or during processing (i.e. poor scorch safety) prior to forming the vulcanized article. The second problem is the inconsistency of the vulcanization rate. It is desired that the elastomer compound, after preparation, be stable under typical environmental conditions of storage and during processing of the compound such that minimal vulcanization occurs prior to formation of the finished article. A consistent vulcanization rate is desired so that processes that form the finished vulcanized article, e.g. injection molding, extrusion, or compression molding can be run consistently and without the generation of scrap materials.
U.S. Pat. No. 4,745,147 discloses curable compositions of a chlorinated polyethylene, a polymercapto crosslinking agent, an acid acceptor, and a polyhydric alcohol. The composition must also contain a cure initiator such as an amine, a quaternary ammonium salt or a quaternary phosphonium salt (collectively hereinafter referred to as “onium salt”). The onium salts are of either the formula R1R2R3R4Z+Cl− or the chloride salt of an N-substituted pyridinium ion, wherein Z is N or P; R1-R4 may be the same or different from each other and selected from C1-17 alkyl, cyclohexyl, phenyl and benzyl groups. The polyhydric alcohol is said to improve the cure state and heat aging of crosslinked compositions.
U.S. Pat. Nos. 5,665,830 and 5,686,537 disclose that variability in the premature curing (scorch) of the 2,5-dimercapto-1,3,4-thiadiazole, amine/quaternary ammonium salt, metal oxide cure system is due to the influence of water and that the variation can be eliminated by addition of polyethylene glycol or glycerin.
U.S. Pat. No. 4,482,681 discloses polymercapto cure systems containing an organic or inorganic base, and a hydrated salt, which releases its water during curing. The hydrated salt improves the cure rate. Some chlorinated elastomers also require an accelerator, which may be an amine or a quaternary ammonium salt.
Despite the research efforts in developing and improving in the scorch safety and cure rate reproducibility, there is still a need for further improvement in scorch safety while maintaining an acceptable cure rate.